The present invention relates to a certain number of improvements made to a drier for wood veneers, of the type featuring overlaid belts.
As is known to persons with ordinary skill in the art, wood veneers are dried by the combined effect of pressure and heat inside driers designed for the purpose.
The pressure can be applied either by passing the wood veneers through successive pairs of opposed pressure rollers, which also move the veneers forwards, or by disposing the veneers between two overlaid belts which move along a rectilinear path within the drier.
The driers featuring pressure rollers produce a more pronounced flattening than the belt-type driers.
However, because of the elevated pressures entailed, roller-type driers do not lend themselves satisfactorily to the treatment of thin veneers made from delicate, superior woods. In addition, on account of their scant solidity, the thin veneers have to be guided on their path from one pair of rollers to the next, which fact prevents them from shrinking freely and causes flaws to occur in them.
The driers with overlaid belts, on the other hand, with a simple rectilinear path, have the disadvantage that the weight of the upper belt may not suffice to ensure that the veneers are satisfactorily flattened.
To overcome the aforesaid disadvantage of belt-type driers, it has already been proposed to cause the belts to take a generally sinusoidal path instead of a rectilinear one, and a drier of this type is for example described and illustrated in German patent No. 12 66 233.
As a result of the sinusoidal path they follow, the belts are enabled to bring an adequate flattening pressure onto the veneers; however, such pressure is permanent throughout the drying cycle and does not permit the veneers to shrink freely without risk of causing flaws or cracks.
For the veneers are at every instant obliged to follow a curvilinear path during which they are practically always in contact with the guide rollers on which the overlaid belts containing the veneers wind.
A proposal for overcoming this drawback is contained in European patent No. 0152576, and it provides, between one guide roller and the next -the axes of rotation of which are all in one plane- for a rectilinear section of length equal to at least half of the maximum width of the veneers.
In this manner the veneers follow ample curvilinear sections around the relatively large-diameter guide rollers alternated with rectilinear sections between one guide roller and the next.
Such proposal is not however satisfactory; for if it is true that the veneers are free to shrink in the appropriately sized rectilinear section between one guide roller and the next, in the curvilinear sections having a relatively high width, the pressure brought to bear, at least in respect of certain very thin special and also superior wood veneers, can in any case be such as to cause micro-fissures which cause the surface porosity of the veneers to vary, thus creating problems in the subsequent lacquering operations.
In addition, the arrangement of all the guide rollers in one and the same plane represents a very considerable limitation as regards mounting nozzles able to blow warm air directly onto the veneers contained between the overlaid belts. Because of this, the drying process becomes relatively lengthy, unless costly heating elements are provided within each roller.
The overall object of the present invention is to obviate the aforesaid drawbacks of the known art by embodying a drier for wood veneers, of the type with overlaid belts, in which the path of the belts is such as to allow application of an adequate flattening pressure also to thin, high-quality wood veneers, to allow a shrinkage without fissuring, and to permit a straightforward and economical mounting of nozzles able to blow warm air directly onto the veneers, so as to speed-up the drying process.